Question

Why in lenses where c is mentioned there 2f is also menstio?

Answer 1

It's so because radius is double of focus that is f and c is centre of curvature then line joining c and p will be called radius of curvature and
r
$f = r \div 2$
$r = 2f$
and r is equal to distance between c and p so the distance of c from p is 2f that's why it is mentioned as 2f.